1. Field of the Invention
The present invention relates to driving LEDs (light-emitting diodes) and, more specifically, to a system for driving multiple strings of LEDs.
2. Description of the Related Arts
LEDs are being adopted in a wide variety of electronics applications, for example, architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices including personal computer, laptops, high definition TVs, flashlights, etc. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety.
LEDs are current-driven devices, meaning that the luminous flux (i.e. brightness) generated from them is primarily a function of the current applied through them. Thus regulating the current through the LEDs is an important control technique. To drive a large array of LEDs from a direct current (DC) voltage source, DC-DC switching power converters such as a boost or buck-boost power converters are often used to supply the top rail voltage for several strings of LEDs. In Liquid Crystal Display (LCD) applications using LED backlights, it is often necessary for a controller to control several strings of LEDs in parallel with independent current settings for each string. The controller can then independently control the brightness of different sections of the LCD. Furthermore, the controller can turn different parts of the LCD on or off in a timed manner.
Due to manufacturing differences between the LEDs, the voltage drop across each LED string necessary to maintain a specified current level varies considerably. The VI curve of FIG. 1 illustrates the exponential relationships between voltage and current for two different LEDs (LED1 and LED2). For LED1 and LED2 to provide the same amount of peak current, LED1 must operate at a forward voltage drop of about 3.06 volts, while LED2 must operate at a forward voltage drop of about 3.26 volts. Assuming there are 10 LEDs having the characteristics of LED1 in a first LED string, there is a 30.6 V drop across the string. Assuming there are 10 LEDs having the characteristics of LED2 in a second LED string 102, there is a 32.6 V drop across the second LED string. This difference of 2 volts will therefore be dissipated by circuitry driving the second string such that both strings operate at the same peak current of 40 mA.
The unpredictable VI characteristics of different LEDs makes it difficult to operate different LED strings in a power efficient manner while still maintaining precise control over the brightness of the LED strings. Different techniques have been developed to address this challenge, but many conventional solutions are either inefficient or require the use of additional circuitry that substantially increases the cost of the components used to regulate current through the LED strings.